It doesn't show up because it gets parsed as (invalid) HTML. You can use HTML to format your post, and it doesn't get shown if it's used for formatting. Here's the list of valid HTML tags. As that page says,

HTML tags not on this list are stripped from the output.

Notice that, for example, <html> is not on that list. If you have a pair of <html> tags at the start and end of your sample, they will be stripped, and your sample will display nothing at all.

You need to tell the parser not to render the sample, and instead to display it. You can do that in one of three ways:

For short, inline samples like my use of <html> above, use backticks (next to the 1 on your keyboard). The above line looks like:

my use of `<html>` above

For longer samples, use block code formatting. To do this, indent every line of the block by 4 spaces. You can do this manually, in a text editor, or by selecting the text and hitting the {} button in the toolbar:

Remember that you can use HTML to format your post. Enclose your code in <pre><code> to invoke the syntax highlighter and format your text as code.